Family Xyalidae

Rev. 07/26/2025  

Nematoda

Chromadorea

Chromadoria

Monhysterida

Monhysterina

Sphaerolaimoidea

Xyalidae Chitwood, 1951

	Various configurations of cephalic setae in family Xyalidae: Characteristic 1: Inner labial seta visible or invisble Characteristic 2: Outer labial setae prwesent or absent Characteristic 3: either zero, 4, or 8 cepalic setae Characteristic 4: zero or 1 supplementary cephalic setae adapted from Nicholas and Trueman, 2002
Pseudosteineria sp (Xyalidae): Cephalic setae (from Cesaroni et al., 2017)

• A widespread family of mostly marine free-living nematodes.
• Free-living nematodes of marine and brackish water; often among the most abundant of the nematode taxa in an environment..
• Body length 0.5-2 mm.
• Cuticle finely striated and with long somatic setae.
• Ocelli sometimes present.
• Mouth cavity simple, funnel-shaped, denticles either absent or minute.
• 6+4 sensillae almost always situated very close, for example, the 2nd  annule with 6 cephalic setae anf the third with 4.  There may be with additional cephalic setae.
• Esophagus cylindrical.
• Female monodelphic prodelphic, vulva at 60-65% of body length. Per Lorenzen (1994, in didelphic forms,  the anterior gonad is always on the left of the intestine and the posterior gonad on the right.
• Male with two testes, anterior teatis on left and posterior teatis on right side of the intestine.
• Males occur commonly in most genera.
• Circular amphids, some marine forms with spiral amphids.
• Tail elongate conoid, terminated in a short spinneret..
• Many of the species descriptions of nematodes made in the 20th century were made based on very few specimens with relatively few characters of taxonomic value (Venekey et al., 2014). Consequently, taxonomic reviews and revisions are difficult,

The Xyalidae, are considered, with the Sphaerolaimidae to be in the superfamily Sphaerolaimoidea of the Order Monhysterida. Other superfamilies in the Monhysterida are Monhysteroidea (family Monhysteridae) and Siphonolaimoidea (families Siphonolaimidae and Linhomoeidae). One usually consistent feature is the possesion  of a single out-stretched ovary in female. However, assignment of families to superfamilies varies with authors and through time  (Lorenzen, 1978; Lorenzen, 1994; Nicholas and Trueman, 2002).

References:

Cesaroni, L. Guidi, L., Balsamo, M., Semprucci, F. 2017. Scanning electron microscopy in the taxonomical study of free-living marine nematodes. Microscopie 28:6970

Fonseca, G., Bezerra, T.M. 2014. 7.14 Order Monhysterida in A. Schmidt-Rhaesa. Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifers, Vol 2. Nematoda. De Gruyter, Berlin. 759p

Lorenzen, S. 1977. Revision det Xyalidae (freilebende Nematoden) auf der Grundlage einer kritischen Analyse von 56 Arten aus Nord- und Ostsee. Veroff Inst Meeresforsch Bremerh 16:197-261.

Lorenzen, S. 1978. The system of the Monhysteroidea (Nematodes) – a new approach. Zoologishe Jahrbücher Systematik 105: 515-536.

Lorenzen S 1994. The phylogenetic systematics of free-living nematodes.The Ray Society Institute, London, n.162

Nicholas, W.L. and Trueman, J.W.H. 2002. The taxonomy of the family Xyalidae Chitwood, 1951 (Monhysterida: Nematoda): a cladistic analysis. Nematology 4:453-470.

Venekey, V., Gheller, P.F., Maria , T.F., Brustolin, M.C., Kandratavicius , N., Vieira , D.C., Brito, S., Souza, G.S., Fonseca, G. 2014. The state of the art of Xyalidae (Nematoda, Monhysterida) with reference to the Brazilian records. Marine Biodiversity DOI 10.1007/s12526-014-0226-3

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